1. Field of the Invention
The invention relates to the fabrication of irradiation targets used in the production of radioactive products, particularly certain radioactive ions and radioisotopes.
2. Description of Related Art
Ceramic compositions have been used in fabricating irradiation targets that provide increased beam penetration, thereby allowing a given beam to penetrate a greater number of targets and providing a corresponding increase in the yields of the desired radioactive products, particularly radioactive ions and radioisotopes. In addition to allowing increased beam penetration, the porosity of the ceramic compositions tends to provide improved ionic effusion and diffusion rates relative to other target materials.
Slip casting, a conventional method used for manufacturing ceramic targets, is affected by many variables including the rheological properties of the slurry. For example, the properties of a particular composition will depend on the specific combination of binders, organic additives, solvents and ceramic powders used to form the composition. Similarly, the mechanical processing operations applied to the composition may affect the properties of the composition. The composition and rheological properties of the slurry, as well as the post-casting treatment, will, in turn, determine the properties of the final cast product.
For example, the viscosity of the slurry and ceramic particle size distribution are affected by the solubility of the binder and presence and composition of additives such as dispersants. The solids content of the slurry, also referred to alternatively as a slip or a suspension, is an important factor in determining the density and lateral shrinkage as detailed in A. Tsetsekou et al.'s article entitled “Optimization of the Rheological Properties of Alumina Slurries for Ceramic Processing Applications Part I: Slip Casting,” Journal of European Ceramic Society, Vol. 21, Issue 3, March 2001, pages 363-73, the disclosure of which is hereby incorporated by reference, in its entirety. Higher densities tend to be associated with fewer and smaller pores and improved bonding between the particles which, in turn, tends to provide improved heat conduction to the surrounding material. Improved heat conduction is useful in, for example, allowing higher beam energies to be applied to the ceramic material without causing excessive localized heating or vaporization of the target material.
Large pores and/or bubbles in the green castings results in a lowered sintered density for the target product. As explained in Xin Xu et al.'s article in J. Am. Ceram. Soc., 86[2] pp. 366-68, 2003, “α-SiAlON Ceramics Obtained by Slip Casting and Pressureless Sintering,” the disclosure of which is hereby incorporated by reference, in its entirety, a smaller pore distribution tends to provide better reactivity between particulates. Two factors that contribute significantly to the green density of the castings are the particle size distribution and the viscosity of the slurry at the time it is cast. Controlling these factors through selection of the appropriate particle size distributions and the composition and content of the liquid portions of the slurry will generally allow the green density of the casting to be maintained within a desired range.
Improved particle dispersion tends to be associated with higher viscosities, but the higher viscosities affect the slip flow behavior and can, therefore, increase the difficulty of forming a casting having a uniform thickness. The slurry viscosity tends to increase with higher solids loading and slurries with higher solids contents also tend to be dependent on dispersant concentration. It has been found that relatively low levels of dispersant, such as between about 0.5 and 2.0 wt %, particularly compositions including about 1.0 to about 1.5 wt %, are useful in preparing high-solids slurries that exhibit an acceptable combination of properties.
Higher slurry viscosities, however, tend to increase the likelihood of trapping bubbles within the casting which will tend to increase the pore sizes in the green casting and reduce its sintered density. Another known factor that that can affect the viscosity of the slurry composition is milling. While traditional mechanical stirring tends not to affect the green density of the stirred composition, milling tends to produce sheer thinning and tends to reduce both the average particle size and the particle size distribution. These milling effects are generally attributed to the improved breakdown of larger agglomerates of the particles and particle-to-particle contact that tends to reduce the average particle size, thereby producing a composition having a higher green density.
A secondary force that plays a role in particle segregation is gravity, as discussed in S. M. Olhero and J. M. F. Ferriera, Ceramics Int'l, Vol. 28, Issue 4, pp. 377-86, 2002, “Particle Segregation Phenomena Occurring During the Slip Casting Process,” the disclosure of which is hereby incorporated by reference, in its entirety. As the particles settle out in the cast there tends to be some segregation of the particles with the finer particles concentrated toward the upper surface and the larger particles tending to concentrate toward the middle of the layer. The lower portion of the cast layer tends to include a mixture of particle sizes.
Total solids loading and amount of fine particles present within the slurry composition will also affect particle packing. Finer particles display higher viscosities with a sheer thinning behavior. This behavior is referred to as a pseudoplastic effect and depends on both on particle orientation and flocculation. Coarser particles tend to have a lower viscosity and a sheer thickening effect. A larger particle size distribution will increase the overall green density, as the finer particles will fill in the gaps between the larger particles, thus allowing for a better packing order.
Another issue to be considered, in particular with aqueous slip casting, is the solubility of organic additives. The preferred binder compositions will be those that may be dissolved in an aqueous solution or that may be prepared as a fine aqueous emulsion or suspension. Generally, a range of polymeric emulsion binders may be used successfully to prepare slurry compositions having the desired rheological properties. The binders selected will generally have a higher viscosity, particularly for slurries that have a high solids content, and will tend to exhibit better particle suspension and dispersion within the resulting slurry.
Keeping the organic additives content low will also tend to reduce the formation of defects in the green casting. In general, castings having a higher “green” density will experience less lateral shrinkage during the subsequent sintering process. According to Bitterlich et al., specifically B. Bitterlich, C. Lutz, and A. Roosen, Ceramics Int'l, Vol 28, Issue 6, 2002, pp. 675-83, “Rheological Characterization of Water Based Slurries for the Tape Casting Process,” the disclosure of which is hereby incorporated by reference, in its entirety, a high powder to binder ratio tends to improve green density. As used herein, “green” refers to compositions that although dry, have not been subjected to a sintering or densification process.
However, aqueous slurries tend to be dependent on pH. The pH of the slurry controls the surface charge of the particles, which in turn affects the amount of dispersion of the particles and/or the manner in which the particles flocculate within a suspension. The interaction of the particulate matter in the slurry is dependent on the pH value, type of ceramic and charge of organic additives. R. R. Rao et al., specifically R. Ramachandra Rao, H. N. Roopa, T. S. Kannan, Ceramics Int'l 25, 1999, pp. 223-30. “Effect of pH on the Dispersability of Silicon Carbide powders in Aqueous Media” the disclosure of which is hereby incorporated by reference, in its entirety, found that the optimum pH for dispersing SiC particles in deionized water to be basic with a target pH of about 10. Rao et al. also noted that the SiC particles tended to flocculate substantially under a more acidic pH and tended to agglomerate at pH levels above about 10. A suspension or slip that has a high solids loading and an appropriate pH will generally produce castings having improved green density and packing uniformity than compositions having lower loadings and/or less desirably pH values.
Provided that the pH is maintained at a somewhat basic level, the particles will typically tend to stay suspended longer in solution whereas under acidic conditions the rate of particle deposition or segregation tends to increase considerably. It also seems that the amount of dispersing agent directly affects how well the particulates stay suspended in solution. See Ceramic Microstructures by Electrophoretic Deposition of Colloidal Suspensions, H. von Both, J. Hauβelt, Proceedings of International Conference on Electrophoretic Deposition: Fundamentals and Applications, Aug. 18-22, 2002, J. of Electrochemical Society, the contents of which are incorporated herein, by reference, in their entirety. Although the casting green density tends to be relatively independent of the dispersing agent content, excessive amounts of dispersing agent will tend to slow the manufacturing operation because the particles will tend to remain suspended for a longer period of time and insufficient amounts of dispersing agent will tend to reduce the uniformity of the casting because the particles will tend to fall out of suspension too quickly.